Miners have more coins then everyone else so they are the one who define the price (they won't sell if it's not profitable for them) , beside ... logically Bitcoin price is based on Supply and demand , so It's impossible for the supply to get halved and the demand stays the same or rise and nothing at all happens to the price ...
The problem with your assumption is that the supply does not magically get halved.. Only the supply of NEW coins gets halved. Compared to the 14 million coins already out there, these new coins are like drops in the ocean.
The only way supply would magically halve is if 7 million coins would disappear or become unspendable, and this is not gonna happen, ever.